The subject application is related to U.S. patent application Ser. No. 09/607,573 filed on Jun. 29, 2000, entitled A TEST AND MEASUREMENT INSTRUMENT HAVING TELECOMMUNICATIONS MASK TESTING CAPABILITY WITH A MASK ZOOM FEATURE, (Letts) assigned to the same assignee as the subject application, and also claiming priority from the above-identified U.S. Provisional application, and to U.S. patent application Ser. No. 09/619,067 filed on Jul. 19, 2000, entitled A TEST AND MEASUREMENT INSTRUMENT HAVING MULTI-CHANNEL TELECOMMUNICATIONS MASK TESTING CAPABILITY, (Letts and Herring), assigned to the same assignee as the subject application, and also claiming priority from the above-identified U.S. Provisional application.
The subject invention generally concerns test and measurement instruments, and in particular concerns those test and measurement instruments employing telecom mask features.
In the telecommunications industry, it is commonplace to perform a test to determine if a particular signal is in compliance with parameters established by national and international communications standards bodies such as ITU-T and ANSI. A primary way to perform such a compliance test is to compare the pulse shape of a waveform acquired by an oscilloscope to a waveform xe2x80x9cmaskxe2x80x9d. The mask defines a pathway having minimum and maximum amplitude values, predetermined bit rate, and defined minimum slope on signal edges (i.e., minimum bandwidth). If the signal under test stays within the pathway boundaries, then the signal passes the test. This kind of test is known as Telecom Mask Testing.
A recent innovation in oscilloscope features has been a xe2x80x9cAUTOSET TO MASKxe2x80x9d function. The AUTOSET TO MASK function automatically sets up the horizontal, vertical, and triggering settings on the oscilloscope to accommodate the expected signal, and overlays a mask on the oscilloscope display. The procedure followed in the operation of an AUTOSET TO MASK function is to set the horizontal and vertical scales to a nominal value, acquire a waveform, and adjust the scale and position of the waveform by adjusting the settings of the input A/D converters, and display the mask. Unfortunately, it has been observed that the AUTOSET TO MASK function sometimes places the mask on the display with an undesired offset in the vertical or horizontal directions. The likelihood of incorrect placement of the mask depends upon several factors including the shape of the signal being acquired and the particular algorithm used to implement the AUTOSET TO MASK.function.
It should be understood that an AUTOSET TO MASK function merely sets up the acquired waveform and displays the mask. It does not check for intrusions into the mask area by the waveform being tested (i.e., violations, or mask hits). Thus, the AUTOSET TO MASK function does not provide correction for such violations.
In this regard, note that telecommunications standards often allow some tolerance in vertical offset for the communications signal. That is, telecommunications standards are usually more concerned (i.e. exhibit tighter tolerances) with respect to other signal characteristics, such as pulse width, and rise time. If the telecom mask is incorrectly placed due to such an offset, an otherwise perfectly acceptable signal (i.e., one that should have passed) will unnecessarily fail when it is later tested for intrusions into the mask area.
One prior art response to this problem was a software solution used in the Tektronix 2400 DITS (Digital Interface Test System), manufactured by Tektronix, Inc., Beaverton, Oreg. In this arrangement, a waveform was acquired by the oscilloscope, rasterized, and stored. Each pixel of the rasterized waveform is indexed to a particular list of vertical segment points occupied by the mask at that particular horizontal (i.e., time) position. Thereafter, the individual pixels of the rasterized waveform were read out and compared, one by one, with the corresponding vertical array of mask segment locations to see if pixels in the mask area and the waveform coincided. If so, a xe2x80x9cmask hitxe2x80x9d was said to have occurred. This purely software solution counted the number of xe2x80x9cmask hitsxe2x80x9d that occurred, and repositioned the mask (horizontally or vertically on the display) until the number of mask hits was reduced to zero. This assumes, of course, that the signal under test did, in fact, correspond to the applicable standard. It is important to note that this prior solution did not operate in real time but rather post-processed the rasterized waveform data.
While this solution does perform extremely well for ensuring compliance with standards for low speed telecommunication signals, it tends to be software intensive and time consuming. What is needed is a faster, less software intensive solution to the problem of properly registering the telecom mask with respect to the signal under test.
Initial mask and waveform positions on a display screen may be determined by an AUTOSET TO MASK function. Upon detection of violations that occur after the AUTOSET TO MASK function is complete, control for further alignment of the waveform under test and the mask is assumed by an AUTOFIT TO MASK function. Comparison of the mask pixels and waveform pixels to detect collision between a waveform pixel and a mask pixel (i.e., a mask violation) is performed substantially in real time, as the pixels are being composited into the raster memory by the rasterizer. In the event of a mask violation, a mask violation signal is generated.
In a further embodiment of the invention, in response to the mask violation signal, the AUTOFIT TO MASK function uses display rasterization to automatically redraw the waveform at a new location, until the waveform fits within the mask.
In yet another embodiment, in response to the mask violation signal, the AUTOFIT TO MASK function automatically redraws the telecom mask at a new location until the waveform fits within the mask.